AmericanChemicalSociety.com

The design scheme of a typical discotic liquid crystal (DLC) material generally entails a central aromatic core embellished with three to eight flexible chains. Of great interest is the class of organic semiconducting DLCs as this class of materials has exciting electronic and optoelectronic properties. DLCs may form efficient π-π columnar stacks, which permit the achievement of high charge-carrier mobilities, whose magnitude is fundamentally determined by the degree of order and π-π molecular orbital overlap within the columnar stacks. The charge transport within the stack, i.e. in a one-dimensional manner, is therefore a built-in feature of the design scheme. Ease of film formation is another key property of smart materials for organic-based electronic and optoelectronic devices. As DLCs have a predisposition to self-organize into columnar stacks, an innate ease of processability is pre‑programmed.

The synthesis of new materials was undertaken under the premise to use these materials in liquid crystalline photovoltaic applications. Discoid compounds 6,7,15,16-tetrakis(alkoxy/alkythio)quinoxalino[2′,3′:9,10]phenanthro[4,5-abc]phenazine (referred to as TQPP-[Y]₂-[XR]₄ (Y= H, t-Bu; X = O, S, R = C_nH_2n+1), Figure 1, were efficiently synthesized and characterized in our labs.

Figure 1. Chemical Structure of TQPP-[Y]₂-[XR]₄.

To evaluate the materials' suitability, a ménage of thermal and spectroscopic techniques was applied to study the phase structures and transitions of TQPP-[SC_nH_2n+1]₄, namely differential scanning calorimetry (DSC), polarized light microscopy (PLM), wide-angle X-ray diffraction (WAXD), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FT-IR). When studying TQPP-[SC₁₂H₂₅]₄ as a model compound using DSC and 1D WAXD, four crystalline and no liquid-crystalline phases were identified. The 2D WAXD experiments on oriented samples of TQPP-[SC₁₂H₂₅]₄ and SAED patterns from single crystals revealed the unit cell of the lowest temperature TQPP-[SC₁₂H₂₅]₄ crystalline phase (K1) to be monoclinic with dimensions of a = 1.87 nm, b =0.53 nm, c = 3.51 nm, and  b = 96.2°. A controlled increase of the temperature, allowed the K₁ phase to be transformed to the other crystalline phases which all were monoclinic with different crystallographic parameters. The various crystalline phases deviated only slightly in the placement of the aromatic TQPP-[SC₁₂H₂₅]₄ core, yet strongly in the conformation of the alkyl chains attached to the rigid cores. Other TQPP-[SC_nH_2n+1]₄ compounds possessed only two crystalline phases wherein the transition temperature can be fine-tuned by modifying the attached alkyl chains to the four corners of the rigid fused rings. Using one-dimensional WAXD studies, solely crystal–crystal transitions for the phase transitions of these compounds were identified. While single crystals of semiconducting materials would provide the highest charge carrier mobility, process costs skyrocket when considering the difficulty to grow single crystals while achieving uniform and large area coverage. The achievement of liquid crystalline phases in these compounds for their application as organic photovoltaic materials hence mandates the modifications to the alkyl chains. We are therefore currently exploiting the synthesis of TQPP-[t-Bu]₂-[SPhytanyl]₄. In addition, the mesophase behavior studies of TQPP-[t-Bu]₂-[XR]₄ (X = O, S, R = C_nH_2n+1) are currently ongoing.

Apart from molecular packing, morphology, and phase behavior, optical and electronic properties such as energy gap and a thorough understanding of light-triggered processes are of vital importance for OPV materials. However, detailed photophysical and photochemical study of discoid compounds remain scarce. The change of the band gap, for example, can be significantly altered synthetically by extension/shortening of the conjugation length and a minimization of bond-length alternation along the conjugation length. We have undertaken an in-depth spectroscopic characterization of TQPP-[t-Bu]₂-[XC_nH_2n+1]₄ (X = O, S). When compared to pyrene, TQPP materials comprise a larger, conjugated core size, yet exhibit a nearly superimposible fluorescence behavior, also in terms of monomer and excimer emission. Despite the observed low solubility for TQPP materials, the solvatochromic behavior of these materials was investigated in a series of solvents from polar to nonpolar, i.e. methanol to cyclohexane. Important photophysical properties such as Stokes shifts, fluorescence lifetimes, fluorescence quantum yields, as well as radiative and nonradiative rate constants were determined for a series of these TQPP materials in various solvents, Table 1. The number of carbons in the alkyl chain R for TQPP-[t-Bu]₂-[OR]₄ and TQPP-[t-Bu]₂-[SR]₄, correlated with the Stokes shift, fluorescence quantum yield, and lifetime. We found a solvent-dependent quantum yield in these compounds and a correlation of monomer to excimer intensity ratio versus gross solvent scale (ET30) and orientation polarizabilty (Δf). Furthermore, while TQPP-[t-Bu]₂-[XR]₄ showed a similar fluorescence and excitation behavior as that of pyrene (Figure 2), a relatively smaller lifetime was observed for these compounds compared to that of pyrene.

Table 1: Monomer lifetime, excimer lifetime, fluorescence quantum yield, radiative and non-radiative rate constants of TQPP-[t-Bu]₂-[XR]₄ in various solvents at 25 °C.

Figure 2. Excitation and emission spectra comparison for pyrene, TQPP-[t-Bu]₂-[OC₁₂H₂₅]₄ and TQPP-[t-Bu]₂-[SC₁₂H₂₅]₄ in THF.